Position measuring device and method for determining a position

ABSTRACT

A method for position determination in a position measuring device that includes digitizing analog position data from a detector unit within time intervals of an internal clock rate and calculating position data from the digitized data. Determining a period of time Δt between a pulse of said internal clock until an appearance of an external trigger signal and a processing of at least two position data, together with the period of time Δt.

[0001] The invention relates to a position measuring device, inparticular for employment in connection with a lithographic system. Theinvention moreover relates to a method for position determination in aposition measuring device.

[0002] It must be possible by means of a position measuring device in alithographic system to determine the position of a movable object insuch a way that the time of the measurement, as well as the position ofthe movable object are very precisely fixed. A lithographic systemoperating in accordance with the scanner principle moves a wafer tableand a photo-masking table, for example, in relation to each other, andin the process exposes only a portion of the area to the photo-mask tobe represented on a wafer. Spatial or chronological errors inpositioning, and therefore in position determination, result in overlayerrors of the structures to be represented with respect to structuresalready present on the wafer.

[0003] In such a lithographic system a central control requests apositional value from a position measuring device at fixed times (forexample every 50 μs), which then must output the position of the movableobject (for example the mask table) at the time of the position valuerequest as the response. Because of the high displacement speedscustomary today (approximately 2 m/s for the mask table), an inaccuracyof one nanosecond in the chronological determination of the positionalvalue then means an error of two nanometers in the positiondetermination, which approximately corresponds to the demands made onsuch a position measuring device.

[0004] It is therefore the object of the invention to disclose aposition measuring device which can determine the position of a movableobject at a time fixed by an exterior source and thereafter can outputit.

[0005] This object is attained by a device having the characteristics ofclaim 1. Advantageous embodiments ensue from the characteristics listedin the claims depending from claim 1.

[0006] It is a further object of the invention to disclose a methodwhich permits the extremely accurate determination of the position of amovable object at the time a position request is made, and subsequentlyto make this available for further processing.

[0007] This object is attained by a method having the characteristics ofclaim 4. Advantageous details ensue from the steps disclosed in theclaims depending from claim 4.

[0008] The principle of the invention is based on determining theposition of the movable object at intervals which are predetermined byan internal clock device of the position measuring device, to determinethe chronological distance of a trigger signal from the last positionmeasurement when a position request (trigger signal) arrives from acontrol unit, and to extrapolate a positional value at the time of thearrival of the trigger signal from at least two previously measuredpositional values. This extrapolated positional value is then output asthe response to the position request.

[0009] Further advantages, as well as details of the present inventionensue from the following description of a preferred embodiment by meansof the drawings. Shown are in:

[0010]FIG. 1, a position measuring device,

[0011]FIG. 2, a diagram showing the chronological sequence of theposition determination.

[0012]FIG. 1 shows a position measuring device 1, whose components andtheir functions will be explained in what follows. Analog signalscorresponding to the physical measuring principle used are generated ina detector unit 2. These can be, for example, signals from aphoto-detector, which receives modulated light from the scanning of anoptical grating measuring system with an incremental graduation andreference pulses. The use of optical measuring principles suggestsitself because of the high resolution required in the above mentionedcase of application (it would also be possible to detect signals from aninterferometer in the detector unit 2), but in principle other measuringmethods are also possible, based on magnetic, inductive or capacitiveeffects, which scan linear, as well as rotatory movements. In the caseof application described, the detector unit 2 detects the linearmovement of a mask table.

[0013] The analog signals from the detector unit 2 are conducted to anA/D converter 3. The latter digitizes the analog signals respectively attimes t preset by an internal clock 6 of 1 MHz at intervals of 1 μs andpasses the digitized values on to a processing unit 4, in which apositional value is calculated from them. The position measurement atthis fast rate is necessary in order to make possible the counting ofthe increments in an incremental measuring system. It is also possibleto perform corrections, for example of the phase position of theindividual digital signals in the processing unit 4, the resolution ofthe position determination can also be increased by interpolation. Anabsolute position determination can be performed by means of theevaluation of reference pulses and the counting of increments of thesignals.

[0014] The positional values obtained in this way are then passed on toan extrapolation unit 5. Reference is made to FIG. 2 in regard to thefurther course of the position measurement.

[0015] If now the position measuring device 1 receives a request(trigger signal 9) for a position measurement from a higher control unit8, the period of time Δt from the last pulse of the internal cycle at tnuntil the arrival of the trigger signal 9 at the time t0 is determinedin a timer 7. The timer 7 can be a time-to-digital converter (TDC), forexample, which is set to 0 by the internal clock 6 and internallymeasures a period of time Δt very exactly, which is digitally outputafter the receipt of the trigger signal 9.

[0016] This period of time Δt is also sent to the extrapolation unit 5.An extrapolation for the position at the time t0 is performed there bymeans of a function which is approximated to at least the two positionvalues P1 and P2 detected at the times tn and tn−1. This has advantages:with an object at rest, a linear extrapolation with two position valuesresults in the mean of the position, and therefore a lowering of thenoise. For an object moved at a constant speed the extrapolation isstill very accurate. For obtaining advantages in regard to noisereduction it is necessary to extrapolate a straight line by means ofthree support points. For accelerated systems it is necessary to fallback on higher order functions (for example polynomials) in order tostill obtain accurate extrapolations. However, if the maximallyoccurring accelerations are known, a limitation to linear extrapolationis possible if he error occurring because of this remains sufficientlysmall. For this purpose, the actual position course P(t) of an objectbeing accelerated in one direction is shown in FIG. 2, together with thetwo positional values P1 and P2 detected at the times tn and tn−1. ΔPshows the error in a position calculation with linear extraplation of astraight line G through P1 and P2. In the described case of application,accelerations of them ask table up to 10 g must be expected, therefore,with an internal clock 5 of 1 MHz, a maximum extrapolation error ΔP of0.05 nm results for Δt=1 μs.

[0017] The positional value determined in this way therefore is anapproximation of the required positional value of the moved object atthe time t0. It is transmitted from the position measuring device 1 tothe higher control unit.

[0018] For optimally utilizing the advantages of noise suppression bytaking the mean, on the one hand, and the exact determination of theposition at the time t0. on the other hand, it would also be conceivableto match the extrapolation flexibly to the respective situation. Thus, alinear extrapolation with many points can take place in case of aslightly accelerated movable object, and a large noise suppression canbe achieved in this way, but with greatly and not constantly acceleratedobjects a fall-back to an extrapolation of higher order can take placefor keeping the deviation ΔP low.

[0019] It is a further advantage of this position measuring device thatit is possible to take signal running times, which otherwise mightfalsify the measured result, into consideration. For example, lightrequires a definite time from a scale to the detector unit 2, theelectronic follow-up devices in the detector unit 2, in the A/Dconverter 3 and in the processing unit 4 further contribute to delays inthe measuring signal. These effects can be taken into consideration byan offset of the period of time Δt determined in the timer 7.

1. A position measuring device for determining the position of a movedobject, having a detector unit (2), at least one A/D converter (3)controlled by an internal clock (6), a processing unit (4), and anextrapolation unit (5), and in addition a timer (7) for detecting theperiod of time Δt between a pulse of the internal clock (6) and anexternal trigger signal (9).
 2. The position measuring device inaccordance with claim 1, wherein the extrapolation unit (5) containsmeans permitting the extrapolation of a position from at least twopreviously measured position data (P1, P2) and the period of time Δt. 3.The position measuring device in accordance with claim 1, wherein thedetector unit (2) is a scanning head of an optical grating measuringsystem.
 4. A method for position determination in a position measuringdevice, containing the following steps: digitizing analog position datafrom a detector unit (2) in an A/D converter (3) within time intervalsof an internal clock rate (6), calculation of position data from thedigitized data in a processing unit (4), determination of a period oftime Δt between a pulse of the internal clock (6) until the appearanceof an external trigger signal (9) in a timer (7), processing of at leasttwo position data (P1, P2), together with the period of time Δt in anextrapolation unit (5).
 5. The method in accordance with claim 4,wherein the period of time Δt is measured by a timer (7), which is resetby the internal clock (6) and which, following the receipt of thetrigger signal (9), outputs the period of time Δt elapsed since the lastreset to the extrapolation unit (5).
 6. The method in accordance withclaim 4, wherein an extrapolation by means of a polynomial function isperformed in the extrapolation unit (5) in order to calculate apositional value at the time of the arrival of the trigger signal (9).7. The method in accordance with claim 6, wherein for determining thepolynomial function more position values (P1, P2, . . . ) determined atprevious times tn, tn−1, . . . are employed than mathematicallynecessary, and a mean for noise suppression is obtained.
 8. Alithographic system with a position measuring device in accordance withone of claims 1 to
 3. 9. A lithographic system with a position measuringdevice, which operates in accordance with a method of one of claims 4 to7.